The term “nanofiber” summarizes a large family of different “one-dimensional” nanostructures, such as nanowires, nanotubes and other filamentous structures having outer diameters in the nanoscale. Carbon nanofibers are used for reinforcement applications, as electrically conductive fillers, as catalyst support, in nanoelectronic devices, as artificial muscles and as a storage medium for gas or electrical chemical storage. However, different morphologies of carbon nanofibers are preferred for different applications.
Other materials are known that can be synthesized via chemical vapor deposition as nanofibers, which may be suitable for use as nanofiber electron emitters for use in field emissive displays. For example, these materials include metal nanowires, such as bismuth, tungsten and silver, metal oxide nanofibers, such as ZnO, metal sulfide nanofibers, such as Cu2S and MoS2 and other compounds that form nanofiber morphologies, such as gallium nitride, boron nitride, boron carbide nitride, silicon and silicon carbide. In one example, SiC nanofibers may be synthesized by a reaction between carbon nanofibers and silica, and the SiC nanofibers adopt the same morphology as the carbon nanofiber clusters. For example, SiC synthesis is described in “Oriented Silicon Carbide Nanowires: Synthesis and Field Emission Properties,” by Zhengwei Pan et al., Adv. Mater. 2000, 12, No. 16, Aug. 16, 2000, which is incorporated herein by reference in its entirety.
Various methods are used to grow nanofibers, which is used herein to include within its definition nanowires, single-walled nanotubes, multi-walled nanotubes and other nanofiber morphologies. Each of these methods results in characteristically different nanofiber morphologies and nanofiber chemistry, which greatly affects the emission characteristics of the nanofibers. For example, plasma deposition of carbon to form carbon nanotips produces an irregular structure of carbon nanotips extending from a layer of graphitic carbon. See U.S. Patent Application Publication No. US 2002-0084502 A1. It is believed that this process would be difficult to scale up to produce large display devices and would result in instabilities in electron emission of the resulting film. In U.S. Pat. No. 6,100,628, a partially graphitized nanocrystalline material was formed by cathodic arc vapor deposition. The plasma characteristics were responsible for producing the partially graphitized nanocrystalline carbon structures, having a plurality of larger particles embedded within a plurality of smaller particles. However, adherence of the particles was poor, unless the surface was first subjected to carbon ion bombardment at −1,000 volts, thereby creating a porous layer.
In another process, pre-formed carbon nanotubes were sprayed onto a surface and selectively attached to a substrate. A portion of the nanotubes adhered to the surface in a pattern. Then, the remaining carbon nanotubes were removed from the surface of the substrate where no adhesion was made between the nanotubes and the surface. The adhesion strength of the resulting pattern nanotubes was sufficient to exceed the 2a or 2b scale in the ASTM Tape Test No. D3359-97, which is now superseded by ASTM Test No. D3359-02. However, the thickness of the patterned nanotube film was generally 0.1 to 1 micrometer with the ends of the carbon nanotubes being oriented in random directions and free to move under the influence of an applied voltage. Thus, it is believed that such films have inherent instabilities that preclude high current densities and high gap voltages that are desirable for acceptable display brightness.
During field emission, an electron extracted and accelerated by an electric field collides, for example, with a phosphor on the screen, and light is emitted. Local instabilities within the phosphor screen are caused by movement of carbon nanotubes having free ends under an imposed voltage difference across the emission gap. The charged tips of carbon nanofibers are attracted by electrostatic forces toward the anode, changing the gap distance. A reduced gap distance increases the apparent field strength causing localized instabilities, which can damage a field emission display.
Carbon nanofibers may be grown by chemical vapor deposition (CVD). However, carbon nanotubes grown by conventional CVD on a substrate fail to show effective adhesiveness. See U.S. Patent Application Publication No. US 2002-0084502 A1, published Jul. 4, 2002, at column 1, paragraph [0006]. Reportedly, the carbon nanotube films also fail to provide uniformity and stability in electron emission applications. Also, it is generally believed that alignment of nanotubes is necessary to achieve good stability and emission characteristics, but alignment increases process complexity and cost.
There exists a longstanding and unfulfilled need for a low cost, highly stable carbon nanofiber emitter for field emission applications.